Metallic components, which are exposed to high temperature must be protected against heat and corrosion.
Especially for gas turbines with its combustion chamber or its turbine blades or vanes it is common to protect the components with an intermediate, protective MCrAlY layer (M=Fe, Co, Ni), which provides oxidation resistance, and a ceramic thermal barrier coating, which protects the substrate of the metallic component against the heat.
An aluminium oxide layer is formed between the MCrAlY- and the thermal barrier coating due to oxidation.
For a long life term of a coated component it is required to have a good connection between the MCrAlY layer and the thermal barrier coating, which is provided by the bonding of the thermal barrier coating and the oxide layer onto the MCrAlY layer.
If a thermal mismatch between the two interconnecting layers prevails or if the ceramic layer has no good bonding to the aluminium oxide layer formed on the MCrAlY layer, spallation of the thermal barrier coating will occur.
From the U.S. Pat. No. 6,287,644 a continuously graded MCrAlY bond coat is known which has an continuously increasing amount of Chromium, Silicon or Zirconium with increasing distance from the underlying substrate in order to reduce the thermal mismatch between the bond coat and the thermal barrier coating by adjusting the coefficient of thermal expansion.
The U.S. Pat. No. 5,792,521 shows a multi-layered thermal barrier coating.
The U.S. Pat. No. 5,514,482 discloses a thermal barrier coating system for superalloy components which eliminates the MCrAlY layer by using an aluminide coating layer such as NiAl, which must have a sufficiently high thickness in order to obtain its desired properties. Similar is known from the U.S. Pat. No. 6,255,001.
The NiAl layer has the disadvantage, that it is very brittle which leads to early spallation of the onlaying thermal barrier coating.
The EP 1 082 216 B1 shows an MCrAlY layer having they-phase at its outer layer. But the aluminium content is high and this γ-phase of the outer layer is only obtained by re-melting or depositing from a liquid phase in an expensive way, because additional equipment is needed for the process of re-melting or coating with liquid phase.